1. Field of the Invention
The present invention relates to a surface light source device using light emitting diodes (LEDs), and more particularly, to a surface light source device using LEDs having a color conversion structure capable of providing high efficiency.
2. Description of the Related Art
In general, due to low-power driving and excellent light efficiency, semiconductor light emitting diodes (LEDs) are generally applied as liquid crystal display (LCD) backlight, vehicle light, and home lighting. However, LEDs emit light with a limited wavelength. Therefore, it is required a method of obtaining white light needed in surface light source.
There has been a method of providing a light source by combining LEDs having different wavelengths, respectively, complementary to each other. For example, there may be provided an LED assembly obtained by mixing LEDs emitting red, blue, and green light, respectively. On the other hand, LEDs emitting single-wavelength light may be provided as a light source. In this case, desired white light may be obtained by using a phosphor for wavelength conversion.
FIG. 1 is a side cross-sectional view illustrating a conventional white light surface light source device 10 using a phosphor.
Referring to FIG. 1, the white light surface light source device 10 includes a housing 11 having an open top surface and an LED board 12 disposed in the housing 11. A plurality of blue LEDs 15 is arranged on the LED board 12 to be electrically driven.
A color conversion layer 17 including a yellow emitting phosphor such as YAG(Y3Al5O12):Ce is disposed at a top of the housing 11. The color conversion layer 17 provides yellow light Y by converting a part of blue light B emitted from the LEDs 15. The yellow light Y is mixed with blue light B′ that is not converted, thereby finally emitting desired white light.
However, generally, a combination of a yellow emitting phosphor such as YAG:Ce and a blue LED shows low color rendering properties. That is, since a wavelength of the white light obtained by using the yellow emitting phosphor is distributed only in blue and yellow, there is a limitation on embodying desired natural white light due to low color rendering properties.